Dinitrotoluene is an important intermediate in producing toluenediisocyanate based polyurethanes. One of the conventional processes for producing dinitrotoluene is referred to as the mixed acid nitration process wherein toluene is reacted with nitric acid in the presence of sulfuric acid in a mononitration zone to produce the intermediate product, mononitrotoluene (MNT), which then is separated from the spent acid and the recovered mononitrotoluene contacted with nitric acid in the presence of sulfuric acid to form dinitrotoluene in a dinitration zone. Dinitrotoluene (DNT) then is separated from the aqueous phase and the spent sulfuric acid purified and concentrated. Typical, product specifications (weight %) for commercial grade dinitrotoluene generally will be greater than 98.5% dinitrotoluene, less than 0.1% mononitrotoluene, less than 0.1% trinitrotoluene, less than 0.1% by-products, less than 1.0% water and yield of toluene to dinitrotoluene greater than 98% at conversion levels of &gt;99.9%. The isomer content of product DNT contains a minimum of 95% (on a water free basis) 2,4-plus 2,6-isomer with the balance of the DNT being made up of 2,3-; 3,4-; 2,5-; and 3,5-isomers. The normalized 2,4-plus 2,6-isomer content is between 79.0-81.0% 2,4-isomer and between 19.0-21.0% 2,6-isomer.
Commercially, toluene is converted to mononitrotoluene employing a "cycle acid" having an approximate concentration of from 0.5 to 2% by weight nitric plus nitrous acid and from 80 to 84% by weight sulfuric acid (all acid concentrations are on an organic free basis). Cycle acid s the spent sulfuric and nitric acid recovered from the dinitration zone and used for the nitration of toluene in the mononitration zone. The reaction product from the mononitration zone is separated into an organic phase and a spent acid phase that is recovered for purification and concentration to feed acid strength. The spent acid from the mononitrators typically contains from 0.2-1.5% by weight nitric plus nitrous acid and from 70 to 74% by weight sulfuric acid. The organic phase from the mononitration zone which contains a mixture of toluene, MNT, and DNT is fed to the dinitration zone and is contacted with additional mixed acid where the reaction to form DNT is essentially completed. Mononitrotoluene is typically converted to dinitrotoluene at mixed acid reaction concentrations of from 0.5 to 2% by weight nitric plus nitrous acid and from 80 to 84% by weight sulfuric acid.
Two moles of water are produced for each mole of dinitrotoluene produced, Accordingly, the concentation of feed nitric acid and feed sulfuric acid for the plant must be substantially greater than the concentration of nitric and sulfuric acid in the cycle acid. To meet the sulfuric acid requirements of commercial plants and produce commercial grade dinitrotoluene for the mixed acid nitration process, the feed sulfuric acid concentration has been within a range of approximately 93-98% or higher, generally above 95% by weight when used with subazeotropic nitric acid. Subazeotropic nitric acid has a concentration range from about 57 wt. % up to 69 wt. % nitric acid with nitric acid concentrations of 60-65 wt. % being typical for nitric acid used for the production of dinitrotoluene. Concentrations of &gt;69% nitric acid require more costly methods of concentration, such costs are primarily associated with overcoming the azeotrope.
In a commercial DNT plant, the concentration of sulfuric acid in the cycle acid is set by the concentration of the feed sulfuric acid and feed nitric acid and the concentration of sulfuric acid in the spent acid from the mononitration zone. For a given concentration of sulfuric acid in the spent acid obtained from the mononitration zone, the concentration of sulfuric acid in the cycle acid will decrease as the concentration of feed sulfuric acid and the concentration of feed nitric acid decrease. Typically, the concentration of sulfuric acid in the cycle acid will have to be greater than 80 wt. % to yield a spent acid from the mononitration zone having a concentration of sulfuric acid ranging from about 70 to 74% by weight. There are essentially two problems associated with the generation of a spent acid from the mononitration zone having a sulfuric acid greater than about 74%, they are: it requires increased sulfuric acid usage, and it creates problems associated with the purification and concentration of the spent acid to feed sulfuric acid strength. Sulfuric acid concentrations lower than about 70% by weight result in excessive by-product formation and low conversions of toluene to mononitrotoluene.
Spent sulfuric acid recovered from the mononitration zone which has been diluted by virtue of the production of two moles of by-product water in the process has to be purified and concentrated to be reused for nitration. Typically, nitric acid, nitrous acid and organics in the spent acid are removed prior to concentration. One of the conventional ways to effect concentration of the spent sulfuric acid has been via a series of multiple-effect evaporators operating under vacuum. The concentration of spent sulfuric acid imposes substantial energy requirements upon the nitration facility, not to mention the environmental control problems associated with avoidance of volatile sulfur oxide emission from multi-stage evaporators. Control of environmental loss of sulfuric acid becomes more difficult as the concentration of the product sulfuric acid increases. These losses can become increasingly significant at a concentration above 89% sulfuric acid. Capital and operating costs associated with the evaporators and vacuum equipment are high ever for increases of 1% acid strength and greater when producing sulfuric acid of strengths greater than 89%.
Representative patents which show the production of nitroaromatics including dinitrotoluene and the concentration of spent sulfuric acid to feed acid strength are as follows:
U.S. Pat. No. 3,204,000 discloses the continuous, countercurrent nitration of toluene through a plurality of stages to produce trinitrotoluene. In a multi-stage, countercurrent process a mixture of concentrated nitric acid and oleum is fed to the final nitrator to effect final nitration of the toluene. The spent acid which has been diluted by the production of water is cycled to stages upstream of the final nitrator and thus the feed toluene which enters the first nitrator is contacted with the most dilute or weakest acid mixture. To avoid undesired oxidation of trinitrotoluene and dinitrotoluene by nitrous acid in the process, the aqueous phase and the organic phase are separated after intermediate stages of nitration in order to remove nitrous acid.
U.S. Pat. No. 3,087,971 discloses a continuous process for producing nitroaromatics coupled with a simultaneous reduction in by-product due to oxidation losses. The process is conducted such that the sulfuric acid concentration is maintained within a level of from 86-95%, the nitric acid is maintained within a range of about 14% and the water is maintained within a range of from 0-13.5% on a weight basis. Dinitration is effected in at least two steps in which from 60 to 75% of the sulfuric acid requirements and up to 20% but not more than 30% of the nitric acid requirements for trinitrotoluene are available for the last step. In the last step where trinitrotoluene is produced, the balance of the nitric acid and sulfuric acid is introduced as a mixture of concentrated nitric and oleum with at least 20% free SO.sub.3. Thus, the trinitrotoluene product is withdrawn in an acid mixture nearly free of water. The feed acid to the facility to maintain reactor concentration is oleum containing 25% free SO.sub.3.
U.S. Pat. No. 2,475,095 discloses a continuous process for the production of trinitrotoluene with the nitration process divided into three divisions. In the first division mononitrotoluene is formed, in the second division the mononitrotoluene is converted to a mononitro/dinitrotoluene mixture, and in the third stage the mixture of mono and dinitrotoluene is converted into trinitrotoluene. A composition at the end of the first stage is shown to comprise 71% H.sub.2 SO.sub.4, 25% H.sub.2 O.sub.1, 1% HNO.sub.3, in the second stage the concentration is shown to comprise 78% H.sub.2 SO.sub.4, 15% H.sub.2 O.sub.1, 2% HNO.sub.3, and 3% nitro-products. At the end of the third stage the concentration is shown to comprise 81% H.sub.2 SO.sub.4, 3% H.sub.2 O.sub.1, 6% HNO.sub.3 and 7% dissolved trinitrotoluene. Mixed acids having at least 30% free SO.sub.3 and at least 30% nitric acid are preferred as the feed acid starting material.
U.S. Pat. No. 3,157,706 discloses a process for producing dinitrotoluene wherein it was found that if the concentration of sulfuric acid was higher than 65%, trinitrotoluene was produced. The process comprises nitrating mononitrotoluene with a mixed acid consisting of 33-36% by weight nitric acid, 60-65% by weight sulfuric acid and the remainder water.
U.S. Pat. No. 2,947,791 discloses a process for producing both mono and dinitrotoluene by the mixed acid nitration method. In that process toluene is continuously nitrated to form mononitrotoluene in a two reactor system which then is subsequently nitrated to a dinitrotoluene product in the form of a mixture containing approximately an 80:20 ratio of 2:4-/2:6-dinitrotoluene isomers. The nitrating mixture contains about 50-60% sulfuric acid, 20-40% nitric acid and 10-20% water. The mixture is prepared by blending 95% nitric acid with a spent sulfuric acid solution. The spent sulfuric acid from the second reactor contains from about 62-75% sulfuric acid.
U.S. Pat. No. 4,496,782 discloses a process for recovering nitric acid from the mononitration of toluene wherein toluene is reacted with nitric acid in the presence of sulfuric acid to form mononitrotoluene. To recover the value of the nitric acid in the spent acid, the mononitrotoluene is subsequently adiabatically nitrated in the presence of the spent acid in a dinitration reactor.
U.S. Pat. No. 5,345,012 discloses a process for producing dinitrotoluene by a single stage nitration of toluene under adiabatic conditions. The examples show the nitration of toluene utilizing a mixture of sulfuric acid and nitric acid where the molar ratio of nitric acid to toluene is slightly greater than 2:1. The nitrating acid is comprised of inorganic components consisting of 60-90% by weight sulfuric acid and 1-15% nitric acid.